In manufacturing a large-scale integration (LSI), tungsten has been widely used for MOSFET gate electrodes, source-drain contacts, memory wordlines and the like. A Cu wiring has been mainly used in a multilayer wiring process. However, Cu has a poor heat resistance and is easily diffused. As such, tungsten has been used for a portion that requires a heat resistance or a portion of which an electric property may deteriorate due to the diffusion of Cu.
A physical vapor film formation (PVD) method has been used as a film forming process of a tungsten film. However, it is difficult to use such a PVD method for portion that requires a high coverage rate (step coverage). Because of this, a chemical vapor film formation (CVD) method which provides good step coverage has been performed to form the tungsten film.
As a method of forming a tungsten film (CVD-tungsten film) using such a CVD method, a method of inducing a reaction of WF6+3H2→W+6HF on a semiconductor wafer as a target substrate, by using a tungsten hexafluoride (WF6) as a raw material gas and an H2 gas as a reduction gas, is generally used.
On the other hand, when the tungsten film is formed by the CVD method using the WF6 gas, fluorine contained in WF6 reduces a gate insulation film in a semiconductor device, particularly in gate electrodes, memory wordlines or the like, which deteriorates an electric property of the semiconductor device. A method of forming a CVD-tungsten film containing no fluorine is under consideration as a way to address such a problem.
As a raw material gas used in forming the CVD-tungsten film containing no fluorine, tungsten hexachloride (WCl6) is known. Although chlorine has a reduction property like fluorine, reactivity of chlorine is weaker than that of fluorine. As such, chlorine is expected to hardly affect the electric property.
In recent years, as the semiconductor device becomes finer and finer, it is difficult to use the CVD method, which is known to provide good step coverage, to bury a film into a complex-shaped pattern. Thus, from the viewpoint of obtaining higher step coverage, an atomic layer film formation (ALD) method which sequentially supplies a raw material gas and a reduction gas while performing a purge process in the course of sequentially supplying the raw material gas and the reduction gas, is getting a lot of attention.
In some instances, when a metal film such as a tungsten film is formed by the CVD method or the ALD method, the metal film thus formed tends to be highly-stressed. Such a metal film causes a semiconductor wafer to be bent, which is detrimental in that a desired masking is not achieved in a subsequent process, in manufacturing the semiconductor wafer. For such reason, there is a need for a technology for allowing a metal film such as a tungsten film used in a semiconductor device to be stressed as little as possible.